The present invention relates to an RF (radio frequency) pulse applying method and an MRI (magnetic resonance imaging) apparatus, and more particularly to an RF pulse applying method and an MRI apparatus which can serve to enhance the effects of prior pulses.
Conventionally, there is known a technique by which data collection by applying imaging pulses of an SSFP (steady-state free precession) sequence is repeated after applying a SPEC-IR (spectral-inversion recovery) pulse, and at the same time a pulse having a flip angle half as wide as the flip angle of the imaging pulse is inserted between a SPEC-IR pulse and the following imaging pulse and between an imaging pulse and the following SPEC-IR pulse (see Non-Patent Document 1 for instance).
There is also known a magnetic resonance imaging apparatus which inserts a pulse string to increase the flip angle before applying an imaging pulse (see Patent Document 2 for instance).
There is further known a method of excitation during an NMR examination by which a pulse string to increase the flip angle with a prescribed amplitude curve is inserted before applying an imaging pulse and, after applying the imaging pulse, a pulse string to decrease the flip angle with a prescribed amplitude curve is inserted (see Patent Document 3 for instance).
[Non-Patent Document 1] Magnetic Resonance in Medicine 45: 1075-1080 (2001)
[Patent Document 2] JP-A No. 2004-329268 (Claim 1, FIG. 3)
[Patent Document 3] JP-A No. 2004-237102 ([0036], FIG. 8)
In the prior art described in Patent Document 1 above, magnetization is brought close to its steady state by inserting a pulse having a flip angle half as wide as the flip angle of the imaging pulse between a SPEC-IR pulse and the following imaging pulse and magnetization is returned from its steady state to the equilibrium state by inserting a pulse having a flip angle half as wide as the flip angle of the imaging pulse between the imaging pulse and the following SPEC-IR pulse.
However, this method of inserting a pulse having a flip angle half as wide as the flip angle of the imaging pulse is not very effective for off-resonance magnetization, as described in “Proc. 8th ISMRM., 1:301 (2000)” for instance, both in bringing the magnetization close to its steady state or returning the magnetization from its steady state to the equilibrium state. As a consequence, there arise such problems as a failure to achieve the desired effect on account of the influence of an uneven magnetostatic field or a failure to achieve the desired effect in the tissue of either water or fat on account of a difference in resonance frequency between water and fat.
Incidentally, neither of the techniques described in Patent Document 2 and Patent Document 3 cited above does not concern a pulse sequence in which prior pulses are applied.